1. Field of the Invention
The present invention relates to a wireless communication system, and particularly to a communication apparatus in a MIMO (Multiple Input Multiple Output) system, which conducts wireless communications using a plurality of antennas for transmitting and receiving.
2. Description of the Related Art
A received signal r in a wireless communication is given as a result of integrating the product of a directivity G(θ, φ) of an antenna and a characteristic H(θ, φ) of a propagation path, as shown in the equation below.
                    r        =                              ∫            0            π                    ⁢                                    ∫                              -                π                            π                        ⁢                                          G                ⁡                                  (                                      θ                    ,                    ϕ                                    )                                            ⁢                              H                ⁡                                  (                                      θ                    ,                    ϕ                                    )                                            ⁢                                                          ⁢                              ⅆ                ϕ                            ⁢                                                          ⁢                              ⅆ                θ                                                                        (        1        )            
In the above equation, θ and φ respectively represent an azimuth angle and an elevation angle in the receiving side. Generally, because H(θ, φ) is a variable of natural phenomenon which a human can not control, and expresses an incoming wave distribution which is spatially polarized. Accordingly, it is desired that human controllable G(θ, φ) is spatially uniform (nondirectional antenna) for improving a received power, however, it is not practical to construct such an antenna.
As a result, a case occurs that a directivity of an antenna is null in the direction from which radio waves are coming. In this case, the received power is greatly reduced while received electric field density is high.
In recent years, as a technique of improving frequency utilization efficiency, a MIMO system has gathered interests in the field of a wireless communication, in which communications are conducted by a transmitting side and a receiving side both of which have a plurality of antennas respectively. In the transmitting side in the MIMO system, a spatial multiplexing is conducted by simultaneously transmitting different information from the plurality of transmission antennas at the same frequency. In the receiving side, signals are received by the plurality of reception antennas, and information is extracted by demultiplexing the respective signals.
Then, it is necessary that the respective antennas are arranged such that fading correlations between the antennas are sufficiently low in the receiving side because the spatially multiplexed signals are to be demultiplexed.
FIG. 1 shows a configuration of a radio wave transmission/reception apparatus disclosed in the Patent Document 1 below.
Patent Document 1
Japanese Patent Application Publication No. 2004-312381
This radio wave transmission/reception apparatus 1 comprises four antenna element groups 10a through 10d, and a body 20. The antenna element group 10a comprises antenna elements 101a through 104a, the antenna element group 10b comprises antenna elements 101b through 104b, the antenna element group 10c comprises antenna elements 101c through 104c, and the antenna element group 10d comprises antenna elements 101d through 104d. 
The body 20 comprises a switch unit 201, a signal reception unit 202, an antenna control unit 203, an antenna selection unit 204, and an antenna determination unit 205, and selects a combination from the antenna elements 101a through 104a, 101b through 104b, 101c through 104c, and 101d through 104d such that fading correlations between the antenna elements is the minimum.
However, the above conventional antenna selection method has the problems below.
In the MIMO system, there are some factors that determine characteristic, in addition to the fading correlations between antennas. Accordingly, it is not always the most suitable antenna selection method to select the combination of the antennas based only on a reference of the fading correlation.